1. Field of the Invention
The present invention relates generally to a process for treating a substrate in order to remove undesired material therefrom by exposing the substrate simultaneously to radiation and a dense fluid. More particularly, the present invention relates to methods for removing contaminants from a substrate, for etching a substrate surface, and for detoxifying or decontaminating industrial waste materials.
2. Description of Related Art
In the fabrication of various electronic and optical devices, the surface of the devices often become contaminated with undesired materials, such as organic or inorganic materials, which must be subsequently removed. In addition, in the fabrication of such devices, it is sometimes desirable to etch the surface of the device prior to subjecting it to further processing, such as oxide layer deposition or patterned metal layer formation.
As is known in the art of decontamination, ultraviolet radiation, particularly radiation at 253.7 nanometers (nm) and 184.9 nm, is absorbed by contaminant hydrocarbons and causes dissociation thereof. The fragments produced by this dissociation absorb additional ultraviolet radiation and are further dissociated. Complete dissociation results in the formation of water, carbon dioxide, or nitrogen. Ultraviolet radiation has been used to decompose organic contaminants in water and waste water, as disclosed, for example, in the publication entitled "Investigation into Chemistry of the UV-Ozone Purification Process" by E. Leitis, Jan. 31, 1979, Report No. NSF/RA 790038, National Technical Information Service Accession No. PB 296485. However, organic wastes, either solid or liquid, are not usually miscible with water and cannot be treated by standard UV treatment methods. In such cases, the organic wastes are usually incinerated or buried in a landfill. In addition, this type of photolysis process has been used in combination with oxidation processes to remove organic contaminants from a variety of substrates. Ultraviolet/ozone cleaning has been used to remove organic contaminants from surfaces such as glass, quartz, mica, ceramics, and metals, as described, for example, by L. Zafonte and R. Chiu, in the publication entitled "UV/Ozone Cleaning For Organics Removal on Silicon Wafers," Paper No. 470-19, Proceedings of the SPIE 1984 Microlithography Conference, Mar. 11-16, 1984, Santa Clara, Calif. In such a process, two wavelengths of radiation are used 253.7 nanometers and 184.9 nm. The 253.7 nm radiation is absorbed by the contaminant hydrocarbons and produces bond scission. The resulting fragments are further dissociated by additional exposure to the 253.7 nm radiation, and the final products of this dissociation are water, carbon dioxide, or nitrogen. Concurrently, the 184.9 nm radiation is absorbed by molecular oxygen present in the air environment and dissociates the latter to produce atomic oxygen. The atomic oxygen then combines with additional molecular oxygen present in the environment to form ozone (O.sub.3). The ozone oxidizes the hydrocarbon contaminants to produce carbon dioxide and water as the final products. The ozone also absorbs 253.7 nm radiation and is dissociated into molecular oxygen and atomic oxygen. The latter two species recombine to form ozone. Thus, ozone is continually formed and dissociated.
These conventional ultraviolet/ozone processes for cleaning substrate surfaces have several disadvantages. In order for the ultraviolet/ozone process to be effective, it is essential that the substrate be properly precleaned to remove gross contaminants. The precleaning steps generally use polar and nonpolar solvents, followed by an ultrapure water rinse. The polar solvents remove ionic or inorganic contamination which may not be removed by the ultraviolet/ozone step; and the nonpolar solvents remove all gross organic contaminants. In addition, since the conventional ultraviolet/ozone cleaning process is designed to remove monolayer surface contaminants, the process is presently limited to decontamination of minute levels of hydrocarbons. Any gross inorganic or ionic contaminants which were not removed during the precleaning operations will not be removed by the photosensitized oxidation which occurs during ultraviolet/ozone exposure. Further, all gross hydrocarbon contamination must be removed in a precleaning step since the exposure of gross hydrocarbon contamination to ultraviolet radiation will result in cross-linking and charring. Such polymerization and charring products create a shield for buried layers of contaminants and terminate the cleaning process.
Other problems with the conventional ultraviolet/ozone cleaning are that the process is performed in a gaseous environment at atmospheric pressure or reduced pressure, is based solely on photodegradation, and limited ozone contact, and does not include solvation and fluid transfer mechanisms. The previously described oxidation process occurs in a gaseous environment, which is non-intimate and difficult to stabilize. The previously described dissociation of contaminants takes place only on the surface. In addition, only contaminant by-products which are gaseous are removed. Finally, the conventional ultraviolet/ozone cleaning process is limited in that the surfaces to be cleaned must be in line of sight to the ultraviolet light source for effective photochemical degradation. Thus, surfaces with cavities and holes or surfaces located beneath surface mounted components cannot be effectively cleaned.
Another type of cleaning system involves the use of dense phase gases as a replacement for conventional organic solvents. A dense phase gas is a gas compressed under either supercritical or subcritical conditions to liquid-like densities. These dense gases are referred to as dense fluids. Unlike organic solvents, such as n-hexane, or 1,1,1-trichloroethane, dense phase gas solvents exhibit unique physical and chemical properties such as low surface tension, low viscosity, and variable solute carrying capacity.
The solvent properties of compressed gases is well known. In the late 1800's, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether (J. B. Hannay and H. Hogarth, J.Proc Roy.Soc. (London), 29, p. 324, 1897). By the early 1900's, Buchner discovered that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure (E. A. Buchner, Z.Physik Chem., 54, p. 665, 1906). Within forty years Francis had established a large solubility database for liquified carbon dioxide which showed that many organic compounds were completely miscible (A. W. Francis, J.Phys.Chem., 58, p. 1099, 1954).
In the 1960's there was much research and use of dense gases in the area of chromatography. Supercritical fluids (SCFs) were used as the mobile phase in separating non-volatile chemicals (S. R. Springston and M. Novotny, "Kinetic Optimization of Capillary Supercritical Chromatography using Carbon Dioxide as the Mobile Phase", CHROMATOGRAPHIA, Vol. 14, No. 12, p. 679, December 1981).
Documented industrial applications utilizing dense fluid cleaning include extraction of oil from soybeans (J. P. Friedrich and G. R. List and A. J. Heakin, "Petroleum Free Extracts of Oil from Soybeans", JAOCS, Vol. 59, No. 7, July 1982), decaffination of coffee of pyridines from coal (T. G. Squires, et al, "Supercritical Solvents. Carbon Dioxide Extraction of Retained Pyridine from Pyridine Extracts of Coal", FUEL, Vol. 61, November 1982), extraction of flavorants from hops (R. Vollbrecht, "Extraction of Hops with Supercritical Carbon Dioxide", Chemistry and Industry, 19 June 1982), and regeneration of absorbents (activated carbon) (M. Modell, "Process for Regenerating Absorbents with Supercritical Fluids", U.S. Pat. No. 4,124,528, 7 November 1978).
As the complexity of manufactured devices and structures increases and cleanliness requirements for such devices and structures increase, more effective and more efficient cleaning methods are required. Electro optical devices, lasers and spacecraft assemblies are fabricated from many different types of materials having various internal and external geometrical structures which are generally contaminated with more than one type of contamination. (These highly complex and delicate assemblies can be classified together as "complex hardware".) Consequently, there is a continuing need to provide improved cleaning processes in which both gross and precision cleaning are simultaneously accomplished.
With regard to surface preparation, such as in the fabrication of electronic devices, known methods include both chemical and physical means for removing the surface layer from the substrate prior to deposition. Such methods include, for example, wet chemical etching with aqueous or non-aqueous materials, plasma etching, or ultrasonics. Each of these methods has the disadvantages that it requires expensive equipment, uses solvents, and must be performed as a separate processing step. Thus, there is a continuing need to provide improved methods for substrate surface preparation prior to deposition of a material on the substrate.
With regard to the treatment of industrial waste materials, known methods for treating solid hazardous organic wastes include thermal decomposition, pyrolysis, or UV-peroxidation. These methods have the disadvantage that they require burning, reaction, or stabilization of the solid waste prior to disposal. Thus, there is a continuing need to provide improved methods for the treatment of industrial waste materials to remove or destroy unwanted solid organic or inorganic materials, particularly toxic materials.